The present invention relates to light scanning and, more particularly, to a scanning method and device for providing images.
A variety of techniques are available for providing visual displays of graphic or video images to a user. The most familiar of these techniques involves the use of a cathode ray tube (CRT), such as in television displays and monitor displays, which provide images by scanning electron beams. Generally, in such displays the image is uniformly decomposed into a number of picture elements (e.g., pixels) typically of the same size, whereby imagery information (for example, colors and brightness) is consecutively designated for each picture element by applying a rectangular coordinate system, in which the position is non-optionally decided by horizontal and vertical coordinates.
Such systems suffer from several limitations. CRTs are bulky and consume substantial amounts of power hence being undesirable for small-scale applications such as head-mounted or retinal displays. Additionally, the manufacturing cost of CRTs is proportional to the display area thus making them undesirable for large-scale applications such as large screen video projectors. Moreover, the ability of the CRT within a monitor to produce a proportional number of pixels for a unit screen area decreases as the tube size increases. It is recognized that this limits the ability of larger CRTs to produce fine details.
Flat panel displays, such as liquid crystal displays and field emission displays, may be less bulky and consume less power.
A liquid crystal display (LCD) typically includes a matrix of cells and column cells, where each cell contains a liquid crystal. Upon application of an electric field on a particular cell, the liquid crystal is changed from one alignment state to the other and the transmittance of light therethrough is controlled. The many different combinations of all alignment states in the cells in a matrix allow to design the applied electric field so as to provide an image.
A field emitter display typically includes a matrix of row electrodes and column electrodes, such that each cross point can be addressed by signaling the respective row and column electrodes. Upon a suitable signal, addressed to a specific cross point, an electrical field is formed near the respective electrode electrons are extracted from the electrode by tunneling through the surface potential barrier. Once emitted from the electrode, the electrons are accelerated, redirected and focused so as to impinge on a flat surface. The flat surface is typically coated by fluorescent material which is energetically excited by the impinging electrons. When the excited atoms of the fluorescent material experience a transition to a lower energy level, a light is emitting to the eyes of the viewer.
However, similarly to the CRT, flat panel displays become rather expensive for large-scale applications. Conversely, typical flat panel displays utilize screens that are at least several inches across, hence being less favored for applications in which the display is intended to occupy only a small portion of a user's field of view.
Along with a sufficient supply of video equipment and video software, the demand for a large screen image display apparatus for enjoying powerful images has become intensified in recent years. Attempts have been made to develop large-screen video displays which employ a complex arrangement of lenses for projecting the image on a screen. One such system includes three small diameter CRT light sources for the three primary colors of white light (red, green and blue). The three separate colors produced by the CRTs are converged by an arrangement of lenses to project the image on the screen, which can be substantially larger than the screen obtainable using a CRT. However, the brightness and contrast are poor compared to that of a CRT used for home TV video viewing.
Another system includes a liquid crystal panel (a light valve) which spatially modulates and controls the transmission of the three primary colors of white light (red, green and blue) emitted from a light source. An arrangement of lenses focus the light transmitted by the light valve onto a viewing screen such that the three color images are superimposed to form a multi-color image. Although these projectors have fair resolution, there are other unavoidable problems related to this scheme. The incandescent white light source has a relatively short operating life and generates relatively large amounts of heat. The liquid crystal panel devices cannot be manufactured without some minimum number of defects that, in turn, manifest themselves as permanent image artifacts on the screen regardless of the graphic or video source. Additionally, the use of liquid crystal panel introduces a fixed and permanent resolution to the display device, making it very difficult to adapt the electronics to accept other resolutions for display of graphics and text information
One approach to overcoming many limitations of conventional displays is a display in which the image is reproduced by a light beam scanning instead of the CRT's electron beam scanning. In these systems, the image is reproduced by having light beams scanned in accordance with horizontal and vertical synchronizing signals. This is generally achieved by a scanner or a scanning assembly, such as scanning mirrors or an acousto-optic device, scans a modulated light beam onto a physical screen or directly to the eyes of a viewer.
Scanned beam display systems can be used for a diversity of applications, from small-scale to large scale applications, including, without limitation, head mounted displays, retinal displays, video projectors and the like.
In head mounted displays light from an optical fiber is projected by a scanning device, such as rotating polygonal mirrors to produce an image on an image plane. Head mounted displays are used in various applications, including training applications such as pilot training in simulators. In such applications, there is a need for head mounted projectors having extremely high resolution over a large field-of-view, so as to provide eye-limiting resolution.
A retinal display is an optical device for generating an image upon the retina of an eye. Light is emitted from a light source, collimated through a lens, then passed through a scanner device. The scanning device defines a scanning pattern for the light. The scanned light converges to focus points on an intermediate image plane. As the scanning occurs the focus point moves along the image plane (e.g., in a raster scanning pattern). The light then diverges beyond the plane. An eyepiece is positioned along the light path beyond the intermediate image plane at some desired focal length. An “exit pupil” occurs shortly beyond the eyepiece in an area where a viewer's eye pupil is to be positioned. A viewer looks into the eyepiece to view an image. The eyepiece receives light that is being deflected along a raster pattern. Modulation of the light during the scanning cycle determines the content of the image. For a see-through virtual retinal display a user sees the real world environment around the user, plus the added image of the display projected onto the retina.
It is recognized that the use of miniature devices in general, and microelectromechanical systems (MEMS) in particular, is highly advantageous for providing images via scanning. MEMS are of particular interest because they provide sufficient speed for two-dimensional displays. MEMS are fabricated using integrated circuit batch processing techniques and can range in size from micrometers to millimeters. These systems can control and actuate on the micro scale, and function individually or in arrays to generate effects on the macro scale. The development of miniaturized scanning devices is motivated by the prospects of improved efficiency, reduced cost and enhanced accuracy.
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics which are integrated into a single device or platform (e.g., on a silicon chip). The microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Specific for light scanning applications, MEMS consists of scanning micromirrors fabricated using surface-micromachining technology. Scanning micromirrors have numerous advantages over traditional scanning mirrors. For example, they have smaller size, mass and power consumption, and can be more readily integrated with actuators, electronics, light sources, lenses and other optical elements. Additionally, the use of scanning micromirrors allows for more complete integration of the scanning system, thereby simplifies packaging and reducing the manufacturing cost.
Conventional scanning devices and systems typically employ two moving mirrors, one mirror is used in scanning light beams in a vertical direction and the other mirror is used in scanning light beams in the horizontal direction, according to a synchronizing signal. Mechanically, the image resolution is limited by the number of lines that one mirror can scan during the refresh period of another mirror. Thus, the scanning in the vertical direction is typically done by a mirror workable in a low-frequency region (e.g., linear scan), while the scanning in the horizontal direction is done by a resonant mirror which is capable of high-speed operation. Other systems employ two resonant mirrors so as to allow formation of the image via Lissajous figures.
Moving a mirror quickly through a large angle requires high-force actuators to achieve a high resonant frequency. Many types of mechanical actuators for moving mirrors are known in the art [to this end see, e.g., “MEMS Reliability Assurance Guidelines for Space Applications,” Brian Stark, Ed., Jet Propulsion Laboratory, Pasadena, Calif., 1999, the contents of which are hereby incorporated by reference]. In recent years, advances have been made in the miniaturization of mechanical actuation, inter alia in the field scanning MEMS. The twisting moment necessary for rotating the mirror can be generated by, for example, magnetic actuator, electrostatic actuator, thermal actuator, piezoelectric actuator and the like.
Magnetic actuators typically utilize a loop of current and/or a magnetic material to generate a magnetic field, hence to provide the required twisting moment. Industrial attempts to integrate magnetic actuators within MEMS have encountered difficulties in maintaining optimal values of force, temperature or efficiency.
Electrostatic actuators utilize electrical field to provide the required twisting moment. One known method to generate the electrical field is by applying voltage on a parallel plate capacitor. The disadvantage of this method is nonlinearity of the actuation, in particular when relatively large motions are required. Another type of electrostatic actuation is known as a comb drive actuator. A comb drive actuator typically includes rows arcs of interdigitated fingers, whereby half of the fingers are attached to a fixed element and the other half attach to a movable element. By applying the same polarity voltage to elements the resultant electrostatic force repels the movable element away from the fixed element. Conversely, by applying opposite polarity the elements are attracted.
Thermal actuators utilize heating to produce forces and deflections. One type of thermal actuator includes a layered or laminated cantilevered beam having a free end capable of deflecting via thermal expansion in response to temperature gradients among different layers of the beam. Thermal actuators, however, induce large stresses which can cause severe problems for long term reliability. Another type of thermal actuator exploits an effect known as “shape memory alloy effect” in which certain materials, undergoing reversible phase transition, tend to restore their low temperature phase by exerting strong forces. A major drawback of these devices is a fast wear and fatigue of the shape memory alloys, compared to brittle materials.
Piezoelectric actuators are constructed from a material with a suitable crystalline structure. When an external electrical voltage is applied, a mechanical reaction takes place, which, depending on the crystalline structure and the regions where the electrical voltage is applied, causes a compression or tension in a predetermined direction. One limitation of piezoelectric actuators is that the actuation distance is relatively small, typically no more than a few percents of the total length of the piezoelectric material. In addition, piezoelectric actuators generate a considerable amount of heat which reduces the actuation efficiency.
Irrespective of the mechanism which is responsible to the motion of the mirrors, prior art display systems fail to provide high resolution in both the horizontal and vertical directions as demanded by many applications.
For example, one difficulty with prior art displays is a raster pinch, where, due to differences between the actual scan pattern and the optimal raster scan pattern, successive forward and reverse sweeps of the beam results in unevenly spacing of the pixels at several points of the scan pattern. This uneven spacing can cause the pixels to overlap or can leave a gap between adjacent rows of pixels. Moreover, because image information is typically provided as an array of data, where each location in the array corresponds to a respective position in the ideal raster pattern, the displaced pixel locations can cause image distortion.
There is thus a widely recognized need for, and it would be highly advantageous to have a method and device for providing an image via scanning, devoid of the above limitations.